Centimeter wave absorber



Aug. 25, 1942. E. G. LINDER 7 2,293,339

CENTIMETER WAVE ABSORBER 7 Filed June 25, 1940IIIIIIIIIIIIIIIIIIIIIIIIIIIII I 11111 Patented Aug. 2, i942 STAES PTEoFFica CENTKMETER WAVE ABSORBER Ernest G. Linden, Philadelphia, Pa,assignor to Radio Corporation of America, a corporation of Delaware Thisinvention relates to the absorption of ultra high frequency radio,energy, and has for its principal object the provision of means forpreventing the reflection of radio frequency energy from objects, andfor minimizing standing wave effects due to such reflections.

It is well known that ultra high frequency waves of radio energy,particularly those in the quasi-optical spectrum, are reflected fromconducting and non-conducting surfaces in a manner similar to thereflection of light and sound energy. In a radio transmitter, forexample, ultra high frequency energy is radiated from those parts whichare at a high radio frequency potential. Some of this energyis-reflected back to the transmitter and induces currents in the variouscircuits which may be in or out of phase with the currents normallyflowing therein, or which may have some intermediate phase, depending onthe distance traveled by the reflected wave.

Where the frequency is very high, the distances involved may be" equalto many wavelengths. It will be appreciated, therefore, that thecurrents induced in the circuits by the reflected energy may modify theamplitude of the normal currents considerably. This effect isparticularly serious where the signal source is frequency or amplitudemodulated. In the former case a change of frequency changes the .phaseof the reflected energy, and thus changes the amplitude of the resultantcurrent in the transmitter circuit. The resultant amplitude modulationis not desirable. In the latter case,-the signal is seriously distortedsince the effect of the reflected energy is different for each side bandfrequency. One solution to this difiiculty has been described in acopending application of Rene A. Braden, Serial No. 265,172, filed March31, 1939, in which shields are placed very near all high frequencycircuits so that there can be no substantial phase shift in thereflected energy. The present invention, however, proposes to accomplishthe same result by covering reflecting surfaces with an absorbentmaterial which produces a deadening efi'ect similar to that produced byacoustical absorbing materials.

view of an. absorbing panel;' and Figure 3 is a view of a constructionbrick having high frequency absorbing properties.

Referring now to Fig. 1, an ultra high frequency modulated oscillator 5,or other source of high frequency energy is mounted in a groundedmetallic shielding box 1, closed on all sides, or substantially so, inthe conventional manner. When the dimensions of the box are of the orderof the wavelength, or larger, of the radio frequency oscillator, wavesradiated from the oscillator will cause standing waves to be producedwithin the box. At certain frequencies electrical resonance isestablished with the result that a very high field intensity is producedwithin the enclosed space. This is called a resonant cavity condition,and is well known, The phenomena may be put .to many uses, but it is thepurpose of the present invention to provide means for preventing such acondition from occurring when it is not desired, not meif'iely bydetuning the cavity, since this is only e ective at a particularfrequency, butby absorbing the energy.

For this purpose small masses of fibrous conducting material 9 areinserted more or less at random within the shield 1. It is not necessaryto cover the entire inner surface of the shield with absorbing material,although this may be done, but a sufiicient amount should be used toinsure the desired result. The absorbers themselves are loosely packedfibres of steel, copper,

' or other conducting mate-rial.- I have found that This invention willbe better understood from the following descriptionwhen considered inconnection with the accompanying drawing, and its scope is indicated bythe appended claims.

Referring to the drawing, Figure 1 illustrates a. shielding compartmentwhich includes a high frequency absorbing material to prevent resonantcavity effects; Figure 2 is a cross sectional container whose length maybe varied. A volt meter is mounted within the container to indicate thefield intensity therein. The ratio of the square'root of the maximum tothe minimum field intensity is then determined as the container isadjusted through resonance and anti-resonance. Alternatively thefrequency of the. signal may be varied to determine this ratio. Thisprocess is repeated as the absorbing material is added until there issubstantially no like may be constructed. Test cages are used to shieldradio receivers and the like from signals and electrical disturbancewhich would otherwise make measurements impossible. A cage constructedin the manner suggested would be free from reflections set up by thetest apparatus it-'- self, and the absorbent material will also act asan effective shield against outside disturbances.

Another form herein proposed is illustrated in Fig. 3. In this case theabsorbing fibres are embedded uniformly throughout a brick of insulatingmaterial such as Bakelite, styrol, or any of the well known plastics.The advantages of this arrangement are that the fibres are preventedfrom packing, they are protected from the moisture, which isparticularly important where steel or iron fibres are used, and that thebrick is permanent and easy to handle. The insulating material shouldhave a dielectric constant as near as possible to that of air in orderto minimize reflections which may occur directly from the surface of thebrick. An alternative arrangement would be to coat the individual fibreswith a protective coating of styrol to prevent rusting.

I have thus described a new system for reducing the reflection ofultrahigh frequency energy and for reducing standing waves in a shieldingcontainer. Many further modifications of this invention will becomeapparent to those skilled in the art. -This invention is not to belimited to the specific embodiments which have been herein described butonly by the prior art and the spirit of the appended claims.

I claim as my invention:

1. In an ultra high frequency radio device, a container in whichstanding waves of ultra high radio frequency energy are formed andabsorbing means comprising a mass oi. loosely packe. flbrousconductorspositioned within said con tainer for minimizing said standing waves.

2. In an ultra high frequency radio device, container in which standingwavesof energy a1- formed, and an absorbing material in said con tainerfor minimizing said standing waves, sail material comprising a mass ofconducting fibres 3..In an ultra high frequency radio device, 9.!oscillator, shielding means for said oscillator, am a fibrous conductingmaterial between said oscil lator and said shielding means for absorbingra diations from said oscillator.

4..In a radio device, a source of ultra higl radio frequencyoscillations, shielding means 1'0: said source, said means comprising apacked mas: of fibrous conductors.

5. Ina radio device, a source of ultra high ra- -dio frequencyoscillations, shielding means fOl said source, and absorbing meansbetween saic oscillator and said shield comprised of a loosel: packedmass of fibrous conducting material fol minimizing reflections from saidshield.

6. A non-reflecting shielding material for ultra high radio frequenciescomprising a continuum mass of fibrous conductors enclosin the object tcbe shielded.

'1. An absorbent shield for ultra high radio frequencies comprising acontinuous mass of fibrous conductors covered with an insulatingmaterial enclosing the object to be shielded.

8. In an ultra high frequency radio device, a container in whichstanding waves of energy are formed, and an absorbing material in saidcontainer for minimizing said standing waves. said material comprising amass of conductin fibres of electromagnetic material.

9. In a radio device, a source of ultra high radio frequencyoscillations, shielding means for said source, said means comprising a"packed .mass of fibrous conductors of electromagnetic material.

ERNEST G. LINDER.

